*的字母组合

Given a digit string excluded 01, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

 是一个典型的dfs。 将一个 arraylist 向下传递，达到保存条件的时候，放进去。递归

public class Solution {
    /*
     * @param digits: A digital string
     * @return: all posible letter combinations
     */
     String[] strarr = new String[] {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    public List<String> letterCombinations(String digits) {
        // write your code here
        List<String> ans = new ArrayList<String>();
        if(digits.length() == 0){
            return ans;
        }
        work(digits,0,"",ans);
        return ans;
    }
    
    public void work(String target,int count,String str,List<String> ans){
        if(target.length() == count){
            ans.add(str);
            return;
        }
        int temp = target.charAt(count) - '0';
        if(temp == 0 || temp == 1){
            work(target,count+1,str,ans);
        }else{
            for(int i = 0;i<strarr[temp].length();i++){
                work(target,count+1,str+strarr[temp].charAt(i),ans);
            }
        }
    }
}